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The Description of a Floating Collimator. By Captain Henry Katei% 
F.R.S. Read January 13, 1825. [Phil. Trans. 1825, p. 147.] 

The apparatus described in this paper (of which a drawing is now 
laid on the table) is intended to determine the situation of the line 
of collirnation of a telescope attached to an astronomical circle, with 
respect to the zenith or horizon in some one position of the instru- 
ment ; in other words, to determine the zero point of the divisions on 
the limb. This is at present usually performed by the use of the level 
or the plumb line, or by the reflection of an object from the surface 
of a fluid. The author describes the defects and inconveniencies of 
each of these methods. Those of the plumb line, when applied to 
small instruments (to the improvement of which he describes his at- 
tention to have been particularly directed,) are referrible chiefly to 
want of sufficient delicacy. Those of the level are referrible to a va- 
riety of causes not under the command of the observer ; while ob- 
servations, by reflection, the most perfect perhaps of any now prac- 
tised, require an union of favourable circumstances rarely occurring'. 
Add to these when levels or plumb lines are used, the necessity of 
reversing the instrument, and observing out of the meridian. And 
when observations are made by reflection, that of deferring the cor- 
responding observation to the following night, which has proved so 
great an inconvenience at Greenwich, as to necessitate the erection 
of a second circle for the purpose of simultaneous observation. 

The principles on which the floating collimator is constructed are 
two : the first is the property of a telescope employed by Mr. Gauss, 
and subsequently by Mr. Bessel, in virtue of which the cross wires 
of a telescope adjusted to distinct vision on the stars, may be distinctly 
seen by another telescope, also so adjusted, at whatever distance the 
telescopes may be placed, provided their axes coincide ; the rays di- 
verging from the cross wires of either telescope, emerging parallel 
from its object-glass, and being therefore refracted by that of the 
other telescope to its sidereal focus, as if they came from an infinite 
distance. The author here translates an account by Professor Bessel, 
of a method of using this principle to determine the horizontal or 
zenith point of a circle by the use of a level, employed to place the 
collimating or subsidiary telescope in a horizontal position, a method 
which though characterized by him as the best mode of using a level 
that has yet been devised, is still liable to the objections urged against 
levels in general. 

The other principle which the author substitutes in the place of 
the level, is the invariability with respect to the plane of the horizon 
of a body of determinate figure and weight floating on the surface of 
a fluid. In former inquiries he had satisfied himself that a body floating 
on mercury might be so contrived as to have always, when at rest, the 
same inclination to the horizon. He had thus a floating support to 
which he could attach a telescope, — a support requiring no adjust- 
ment, offering the ready means of extreme accuracy, and precluding 
all fear of those errors which might arise from the use of a level. 


* The collimator in its perfect state consists of a piece of cast iron 
8 inches long, 4 wide, and from -$ to \ an inch thick, having two up- 
rights in the form of Y's, to which the collimating telescope is firmly 
fastened. The support is then floated on mercury in a deal box, 
somewhat larger than the flat portion of the iron, and having its bot- 
tom just covered with mercury. The float is kept in its situation in 
the middle of the box, and prevented from moving horizontally by 
two smooth iron pins projecting from its sides, and moving freely in 
vertical polished grooves of metal let into the sides of the box. The 
whole of the telescope projects above the edges of the box, and a 
screen of black pasteboard with an aperture equal to that of its ob- 
ject-glass, is fixed to the end of the box to keep off false light. The 
instrument was placed on a table attached to the wall of the observa- 
tory, and directed (by looking through the telescope) to the wires of 
a fine achromatic furnished with a wire micrometer. The cross wires 
of the collimator were then illuminated by a small lantern placed 
behind its eye-glass with oiled paper interposed. 

The object of the author in this arrangement being to ascertain 
the limits of variability, in the position assumed by the collimator, it 
was deranged purposely in a variety of ways, by removing and re- 
placing the float, or carrying the whole instrument from its place, 
and every method he could think of used that could fairly introduce 
error. His preliminary trials were made with a wooden float ; but 
this was soon laid aside after ascertaining that the greatest single er- 
ror committed in using it, did not exceed 2"*58 in the position of the 
horizontal point. Other floats were then tried, and it was found that 
the increase of their length and browning their surfaces with nitric 
acid produced material advantages. In 151 single results thus ex- 
perimentally obtained, 28 only were found to give errors in the de- 
termination of the horizontal point exceeding 1", and only two 
amounting to 2". But if the means of every successive 5 be taken, 
and the experiments with the wooden float rejected, the greatest er- 
ror did not exceed 0"'4, and even here the influence of a constant 
source of error depending on the support of the micrometer employed 
was apparent. 

The author then describes at length the mode of using the colli- 
mator and of observing with it. The instrument hitherto described 
may be called the horizontal collimator, but he then proceeds to de- 
scribe a vertical collimator, in which the telescope is fixed perpendi- 
cularly to the float and placed immediately under the axis of the 
circle. By this arrangement the necessity of transporting it from one 
side of the observatory to the other is avoided, the reverse observa- 
tion being made by merely turning the float half round in azimuth. 

It is not necessary that the telescope of the collimator should have 
a tube, nor does the author appear to regard its length as of any im- 
portance, it being merely the direction of its axis which is the subject 
of examination ; and the accuracy of this examination will depend on 
the length and power of the telescope of the circle to be collimated. 


The adjustment of the cross wires in the exact sidereal focus of it* 
object-glass is, however, a point of the highest importance. 

The author next points out an important advantage which this in- 
strument presents, viz. that of enabling the observer, by varying the 
inclination of his float, to detect erroneous divisions of his circle by 
bringing different parts of its arc into use ; after which he proceeds 
to describe an application of his floating collimator, as a permanent 
verification of the verticality of a zenith tube, and considers that by 
its use the error, if any, in the zenith distance of a star, will be ulti- 
mately referred to inaccurate bisection of the star, or imperfections 
in the micrometer screws. 

Notice on the Iguanodon, a newly discovered Fossil Reptile, from the 

Sandstone of Tilgate Forest, in Sussex. By Gideon Mantell, F.L.S. 

and M.G.S. Fellow of the College of Surgeons, fyc. In a Letter to 
• Davies Gilbert, Fsq. M.P. V.P.R.S. fyc. fyc. $c. Communicated 

by D. Gilbert, Esq. Read February 10, 1825. [Phil. Trans. 1825, 

p. 179.] 

The bones of the fossil herbivorous reptile described in this paper 
were discovered in the sandstone of Tilgate Forest in Sussex, which 
is a portion of the iron-sand formation, and forms a chain of hills 
stretching in a W.N.W. direction from Hastings to Horsham. In 
this sandstone the bones and teeth in question are accompanied with 
those of saurian animals, turtles, birds, fishes, shells, and vegetables, 
among which may be satisfactorily traced the remains of a gigantic 
species of Crocodile, of the Megalosaurus, and of the Plesiosaurus. 

The teeth of the three last-mentioned animals are readily recog- 
nised and identified; but in the summer of 1822, others were disco- 
vered in the same strata, which, though evidently referrible to some 
herbivorous reptile, possessed peculiar and striking characters. 
Anxious to ascertain the opinions of naturalists respecting these, the 
author submitted them to the inspection of the most eminent, and 
among the rest to Baron Cuvier, who, while acknowledging that such 
teeth were previously unknown to him, agreed in the conclusion of 
their belonging to some herbivorous reptile of gigantic size, and re- 
commended every research to be made for more connected portions 
of the skeleton. 

Confirmed in his opinion by these remarks, the author renewed his 
researches with increased assiduity ; and though no connected por- 
tions of the skeleton have hitherto rewarded his pains, some of the 
specimens were discovered in so perfect a state as to allow of a com- 
parison with the teeth of recent lacertse in the Museum of the Royal 
College of Surgeons ; and the result of this comparison was, that in 
an Iguana there deposited, teeth w r ere discovered possessing the form 
and structure of the fossil specimens. 

Drawings both of the recent and fossil teeth accompany this paper, 
and w^ere exhibited to the Society. They show a striking corre-